Understanding the molecular interactions that are required for the assembly of the nuclear envelope following mitosis will enhance our understanding of the pathogenesis of cancer as well as other human diseases. Unfortunately, the roles of the various factors involved in the formation of the nuclear envelope are currently not well understood. In particular, a requirement for nuclear lamins (type V intermediate filaments) in the assembly of envelope components, such as membranes and pores, has been suggested by some experiments, and contradicted by others. To further examine the function of lamins in this process, an assay has been developed that uses dominant negative mutant lamins to disrupt lamin function. This assay involves the addition of a fragment from the carboxy terminus of Xenopus lamin B3 (LB3T) to an in vitro assembly system. Preliminary observations indicate that LB3T arrests nuclear envelope formation at a very early stage in the assembly process. Preliminary results also indicate that LB3T can inhibit normal nuclear pore formation, nuclear membrane assembly, and chromatin de-condensation. As a control, the addition of an equimolar amount of a homologous C-terminal fragment from human lamin A (LAT), appears not to have any adverse effects on nuclear envelope assembly. Using LB3T, we will study lamin/lamin dynamics and assess its importance in the assembly of a normal nuclear envelope. Through fractionation of the Xenopus extract, the mechanism of envelope assembly inhibition by the mutant lamin will be addressed, and the minimum effective domain of LB3T that is needed for the inhibition of nuclear envelope assembly will be mapped. With this mutant, the role of lamin polymerization in normal nuclear envelope assembly, specifically nuclear pore and membrane assembly will be elucidated.